1. Field of the Invention
The invention relates to a coating material based on a copper-indium-gallium (CuInGa) alloy, in particular for the production of sputter targets, tubular cathodes and similar coating material sources.
2. Background Art
The production of a CuInGa alloy by ingot metallurgy is in principle known from DE 698 35 851 T2. The composition of the alloy mentioned there substantially equals those of the invention disclosed in the following. EP 0 308 201 A1 relates to a sputter target to be used in the production process of magneto-optical recording media. Correspondingly, U.S. Pat. No. 6,682,636 B2 discloses PVD targets and methods of producing them. However, neither of the two disclosures teaches the use of a CuInGa alloy as a coating material.
Field of application is the production of thin-film solar cells with a semiconductor layer consisting of CuInGaSe2 or CuInGaS2 (common abbreviation CIGS). In this process, an electrically conducting CuInGa layer of the desired composition is deposited, e.g. in a vacuum deposition or sputtering process, onto a molybdenum-coated substrate (KNS glass) In a next process step, the layer is selenized or sulphited in a Se- or S-containing atmosphere so as to preserve its semiconducting properties. The CIGS layer is part of a layer system, made up of a layer package, of a thin-film solar cell.
Deposition of the CuInGa layer has so far been carried out by co-evaporation of elementary evaporation sources (Cu, In, Ga separately) or by vapour deposition of alternating layers of binary master alloys from the Cu—In—Ga system. With these processes, however, constant results with regard to the composition of the layers can hardly be obtained over a longer period of time. One problem is for example a lack of Cu during the use of these methods or a non-uniform distribution of the elements used.
Moreover, several steps are often required for depositing the CIG layer since it is impossible to deposit all three elements by sputtering. The melting point of Ga, for example, is 30° C., which inevitably causes the Ga-metal to melt and drip off during the coating process.
Due to the semiconducting properties of this metal, the electrical conductivity is very low. This, in turn, causes the gallium metal to heat up until the molten phase is reached. Consequently, the coating process is disrupted.
The current state of the art allows for sputter targets, both planar and in the shape of a tubular cathode, to be developed by using the three components Cu, In and Ga in order to create the semiconducting CIGS layers; these sputter targets, however, possess a plurality of infavourable material properties and very poor sputtering and layer properties due to an inappropriate conduction of heat during the production of the ternary coating material.
The following are current methods of depositing the CIGS layer onto different substrates:                When the individual elements are vapour deposited simultaneously, both with and without selenium, at different vapour deposition rates and subsequent tempering, the layer reacts to form a semiconducting chalcopyrite structure.        Vapour deposition (Ga)/sputter deposition (In/Cu) of individual layers with subsequent tempering (and selenization), also by using binary metal targets (CuGa+In or CuIn+Ga) of different compositions.        Sputter deposition of the oxide compounds of the three metals with sub-sequent reduction in, for example, H2 to obtain a layer of pure metal.        Layer deposition based on the Se-containing compounds In2Se3, Ga2Se3, In/Ga2Se3 and Cu2Se.        Magnetron sputtering of the direct semiconducting compound materials CuInSe2 or CuInS2. The efficiency of the solar cells thus obtained is too low.        Further methods involving electroplating or screen printing processes for the deposition of the individual layers are still being tested.        
A sputter target incorporating the three elements Cu, in and Ga at the desired proportion has not been successfully used in any of the above “conventional” methods of production.
The most common method of producing the CIGS layer on an industrial scale is co-sputtering of targets consisting of CuGa (65-80 wt % of Cu) and In. Selenization occurs in a subsequent tempering step at 500° C. in a Se-containing atmosphere (duration approx. 3 min.).
The advantages of the developed CuInGa targets having the desired compositions lie in the fact that a single sputter target is used in the coating process. This results in higher process stabilities and exactly definable stoichiometries, cost reduction and recyclability of the residual coating materials as well as a necessary high melting temperature of the alloy, especially with regard to the element gallium.